This invention relates to apparatus for effecting communication between two or more optical ring networks and, more particularly to optical wavelength-selective cross-connects.
Optical wavelength-selective cross-connects can be used for effecting communications between ring networks and other related applications are typically fabricated using bulk/fiber optics. Another approach is disclosed in U.S. Pat. No. 5,940,551 which shows a wavelength cross-connect for interconnecting nodes made up of a plurality of 2 by 2 wavelength cross-connects. Each 2xc3x972 cross-connect is comprised of two circulators connected together through the medium of a fiber grating that is connected between intermediate ports of the circulators. The fiber grating reflects certain of the wavelength channels in the input signals delivered to the inputs and transmit remaining wavelength channels, thereby enabling each node to communicate with each other node. For planar devices, typical cross-connects are made using two de/multiplex pairs connected by an array of switches for controlling transmission (see K. Okamoto, et. al., Electron. Lett., vol. 32, pp. 1471-1472, 1996, for example). This involves many waveguide crossings which produce high loss, poor crosstalk performance and large device size. Use of a xe2x80x9cdilatedxe2x80x9d switch arrangement (which requires four switches per wavelength) involves even more crossings and the greater number of switches limits the amount of achievable miniaturization and power conservation. While the 2xc3x972 planar cross-connect can be made with crossings it would be extremely advantageous to be able to fabricate a device capable of cross-connecting a large plurality of channels without waveguide crossings.
In accordance with one aspect of the principles of the invention, an illustrative planar waveguide provides an integrated, multi-channel optical cross-connect with improved crosstalk performance, decreased polarization sensitivity, low power consumption and small device size without the need for any waveguide crossings. The intermediate ports of a pair of optical circulators are connected to respective ends of an integrated circuit chip containing a pair of grating multiplexer/demultiplexers driving a respective array of 2xc3x972 Mach-Zehnder interferometers separated by a striped mirror interface and a plate having a thickness of one quarter of the central wavelength on each side of the mirror. The light passes through the device twice making the device polarization insensitive, both in power as well as in wavelength. Any phase errors due to inserting these plates will not cause any disturbances in power, because no interference exists. Because the same router is used for multiplexing and demultiplexing, there are no loss or crosstalk penalties for the through-channels due to mismatch in wavelength response. Back reflections for the switch-condition are suppressed by aligning the waveguide, which passes the striped mirror, under a slight angle so that the reflected light at the interface will penetrate the substrate. To ensure that crosstalk performance is not limited by an imperfect mirror (e.g. Txe2x89xa00), a lateral offset is applied between the ends of the two waveguides with mirrors at the interface. The operating wavelength window of the device can be chosen after fabrication by coupling from the fiber-end directly in the star couplers with two single fibers: no ribbons are necessary.